Hollow workpieces are commonly used in industry and consumer goods. Hollow workpieces with an undercut include preforms for injection stretch blow molding into containers. The containers are usable for aerosol containers, which in turn are usable to dispense air fresheners, shave gel, shave foam, shampoo, body washes, antiperspirants/deodorants, perfumes, hard surface cleaners, etc. Other containers include water bottles, milk jugs, bottles for containing other consumer products, large water jugs, etc. The undercut in the workpiece may comprise threads, a bayonet fitting, etc.
Apparatus for molding hollow workpieces are well known in the art. Such apparatus typically have two mold halves which translate together and apart with relative motion to alternatingly open and close in an axial direction. When closed, the mold halves define a cavity which defines the shape of the workpiece to be formed. If the workpiece has a cavity or void, a core pin may be used. A core pin prevents material to be molded from being in the space occupied by the core pin, allowing a hollow part to be formed.
When the mold halves are opened, the workpiece typically rides with one of the halves. The workpiece is later stripped from the respective mold half. If the mold half has a core pin, the workpiece typically rides with the core pin and is later stripped off while the mold halves are apart.
However, if a workpiece has an undercut the process is not as simple. The workpiece cannot be stripped from the core pin because the core pin material in workpiece undercut prevents the workpiece from axially stripping off the core pin. Such a workpiece may be commonly used as a preform for injection stretch blow molding.
Yet undercuts remain an important design feature of many molded workpieces used for consumer products. Undercuts may occur, for example, in external threads, internal threads, bayonet fittings, snap fittings, grooves, etc.
Various attempts in the art have been tried to make workpieces having an undercut. For example U.S. Pat. No. 3,266,100 uses a complex assembly of collapsible segments. And this attempt makes no provision for a core pin.
One crude attempt to accommodate a core pin for workpieces having an undercut is to simply deflect the workpiece material across the portion of the core pin forming the undercut. But this approach may cause dimensional inconsistency among, and even breakage of, the workpieces. Another approach attempt to accommodate a core pin is to utilize a radially collapsible core. But this approach does not allow for molding of workpieces having a sealing face as the proximal end of the core pin. Even if a sealing face is not required for the desired workpiece, the collapsible core pin requires complex moving workpieces to accommodate the radial motion. Yet another approach utilizes a rotary core pin, so the workpiece is simply unscrewed therefrom. But this approach may require complex rotary indexing if a bayonet fitting is desired and also requires complex moving workpieces to accommodate the additional rotary motion.
The core pin may have an axial counterflow cooling line therein. The core pin may also have one or more hydraulic lines, which may make the core pin a complex assembly. An assembly of a moving core pin and various liquid lines may be susceptible to leaks and entanglement. Thus a core pin which has only axial motion is desirable.
One attempt to utilize a non-rotating and non-collapsible core pin is found in U.S. Pat. Nos. 5,383,780 and 5,798,074. This attempt utilizes a rack and pinion system. In this system, typically a single pinion gear drives plural, typically four, stripper rings. The stripper rings unscrew the respective workpieces. But this system requires a large amount of space and cannot be readily retrofitted to a mold having a conventional axial workpiece ejection system. If a different workpiece is desired, component change-out is difficult. U.S. Pat. No. 6,390,800 claims to unthread a closure for a container as the mold plates are separated. But this attempt also uses a rack gear, which can be costly and complex to operate. U.S. Pat. No. 6,238,202 uses an elongate helically splined spindle, which may have the same disadvantages as an elongate rack.
Thus, there is a need for an apparatus and process which can be retro-fitted to existing core pin molds. The present invention unexpectedly uses existing translation of a stripper plate during the molding process to both open and close one of the mold halves and to release a workpiece with an undercut, eliminating the need for a separate release drive system. The present invention also surprisingly eliminates the need for a large and cumbersome rack to drive the pinion gears. An apparatus according to the present invention having N cavities for molding may have from 1-N different workpieces made thereon, surprisingly providing flexibility not found in the art. Thus the apparatus of the present invention advantageously decouples the number of workpieces which can be made on the apparatus from the number of drive systems necessary to operate the apparatus. The invention is directed to the problem of stripping a workpiece having an undercut without requiring non-axial core pin motion.